AIRCRAFT LANDING GEAR ASSEMBLY

Abstract

An aircraft landing gear assembly (112) including a shock absorber strut (114), a bogie (120), a link assembly (124), and a movement detector (132). The shock absorber strut includes an upper and a lower telescoping parts (116, 118), the upper part being connectable to the airframe of an aircraft and the lower part being connected to the bogie. The link assembly extends between the upper and lower telescoping parts. The movement detector is arranged to detect movement of the link assembly relative to the bogie. The movement detector includes: a piston (338) slidably received within a cylinder (336), fluid which flows as a result of relative movement between the piston and the cylinder; and a pressure transducer (336) arranged to sense a local pressure change in the fluid.

Claims

1. An aircraft landing gear assembly, the aircraft landing gear assembly comprising: a shock absorber strut, a bogie, a link assembly, and a movement detector; wherein the shock absorber strut comprises upper and lower telescoping parts, the upper telescoping part being connectable to the airframe of an aircraft and the lower telescoping part being connected to the bogie such that the bogie may adopt different pitch angles; the link assembly extends between the upper and lower telescoping parts, such that relative movement between the upper and lower telescoping parts causes relative movement between parts of the link assembly; and the movement detector is arranged to detect movement of the link assembly relative to the bogie; wherein the movement detector comprises: a piston slidably received within a cylinder, arranged such that relative movement between the link assembly and the bogie causes relative movement of the piston within the cylinder; fluid which flows as a result of the relative movement between the piston and the cylinder; and one or more pressure transducers arranged to sense a pressure change in the fluid; wherein relative movement between the link assembly and the bogie is detected by the one or more pressure transducers detecting a change in pressure due to movement of the piston within the cylinder.

2. The aircraft landing gear assembly according to claim 1, wherein the cylinder comprises a first chamber, and the first chamber being in fluid communication with a second chamber, and the one or more pressure transducers being arranged to detect when the pressure changes in the first chamber and/or the second chamber.

3. The aircraft landing gear assembly according to claim 2, wherein the cylinder comprises the second chamber, and the first and second chambers being separated by the piston.

4. The aircraft landing gear assembly according to claim 2, wherein the first chamber and the second chamber are in fluid communication by a flow restricted channel; and further comprising: one or more pressure relief channels are arranged to permit (i) fluid flow from the first chamber into the second chamber when the pressure in the first chamber exceeds a first threshold pressure relative to the pressure in the second chamber and (ii) fluid flow from the second chamber into the first chamber when the pressure in the second chamber exceeds a second threshold pressure relative to the pressure in the first chamber.

5. The aircraft landing gear assembly according to claim 4, wherein the movement detector comprises a first pressure transducer being arranged to measure the pressure in the first chamber by measuring the pressure at one side of a flow restricted location within the flow restricted channel and a second pressure transducer being arranged to measure the pressure in the second chamber by measuring the pressure at the other side of the flow restricted location.

6. The aircraft landing gear assembly according to claim 4, wherein the flow restricted channel comprises two flow restricted locations, the one or more pressure transducers being arranged to measure the pressure between the two flow restricted locations; and wherein the movement detector further comprises (i) a first non-return channel connecting the first chamber with the flow restricted channel at a location between the two flow restricted locations, the first non-return channel comprising a non-return valve arranged to only permit fluid flow from the first chamber into the flow restricted channel; and (ii) a second non-return channel connecting the second chamber with the flow restricted channel at a location between the two flow restricted locations, the second non-return channel comprising a non-return valve arranged to only permit fluid flow from the second chamber into the flow restricted channel.

7. The aircraft landing gear assembly according to claim 4, wherein one or more of the following are located in the piston: (i) the one or more pressure relief channels, and (ii) the flow restricted channel.

8. The aircraft landing gear assembly according claim 7, wherein the movement detector further comprising a piston rod connected to the piston, and wherein the one or more pressure transducers are in fluid communication with the first and/or second chambers via one or more channels in the piston rod.

9. The aircraft landing gear assembly according to claim 4, further comprising a body detachably mounted to the cylinder, and one or more of the following being located in the body: (i) the one or more pressure relief channels, and (ii) the flow restricted channel.

10. The aircraft landing gear assembly according to claim 1, wherein the movement detector further comprises: a signal processor being arranged to determine that there is aircraft weight on wheels when the pressure change measured by the one or more pressure transducers exceeds a threshold amount.

11. The aircraft landing gear assembly according claim 10, wherein the signal processor is arranged to generate a binary output indicating whether or not there is aircraft weight on wheels.

12. An aircraft including one or more of the landing gear assembly according to claim 1.

13. A method of detecting aircraft weight on wheels during a landing of an aircraft, wherein the aircraft comprises: a control system and a landing gear assembly; the landing gear assembly comprises: a shock absorber strut, a bogie, a link assembly, and a movement detector; the shock absorber strut comprises an upper and a lower telescoping parts, the upper part being connected to the airframe of an aircraft and the lower part being connected to the bogie such that the bogie may adopt different pitch angles; the link assembly extends between the upper and lower telescoping parts, such that relative movement between the upper and lower telescoping parts causes relative movement between parts of the link assembly; the bogie supports at least one wheel on at least one axle; and wherein the movement detector comprises: a piston slidably received within a cylinder, wherein movement of the piston within the cylinder causes fluid to flow in the movement detector, one or more pressure transducers arranged to sense a local pressure change in the fluid; the method comprising the steps of: the link assembly adopting an initial position relative to the bogie at a point in time that is after the landing gear assembly has been deployed for landing and before the aircraft has touched down; the link assembly moving relative to the bogie during touchdown of the least one wheel thereby causing the piston to move within the cylinder and there to be a transient change in pressure in the fluid; the one or more pressure transducers detecting the change in pressure; the control system receiving a signal from the one or more pressure transducers, the signal being indicative of the change in pressure; and the control system determining, on the basis of the signal, that there is aircraft weight on wheels.

14. The method according to claim 13, wherein the step of the control system determining that there is aircraft weight on wheels comprises the control system determining whether the pressure has exceeded a threshold amount.

15. The method according to claim 13, wherein the signal indicative of a change in pressure is in the form of a pulse.

16. The method of claim 13 further comprising: deploying at least one device to slow the aircraft when the control system determines there is aircraft weight on wheels.

17. (canceled)

18. A movement detector for detecting the weight on wheels condition of an aircraft landing gear, wherein the movement detector comprises: a piston slidably received within a cylinder, the cylinder comprising a first chamber, the first chamber being in fluid communication with a second chamber, and one or more pressure transducers arranged to detect a pressure change in the fluid, wherein, in use, the movement detector is arranged such that movement of the piston relative to the cylinder from a neutral position is caused by movement of parts of the aircraft landing gear caused by one or more wheels contacting the ground, and movement of the piston relative to the cylinder causes a pressure change in the first chamber and/or the second chamber, whereby the weight on wheels condition is, in use, detected by the one or more pressure transducers arranged to detect a pressure change in the fluid.

19. (canceled)

Description

DESCRIPTION OF THE DRAWINGS

[0076] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0077] FIG. 1 shows a side view of an aircraft comprising a landing gear assembly;

[0078] FIG. 2 shows a side view of a prior art landing gear assembly;

[0079] FIG. 3 shows a side view of a landing gear assembly according to a first embodiment of the invention prior to touchdown;

[0080] FIG. 4 shows a side view of a landing gear assembly according to a first embodiment of the invention after touchdown and before shock absorber compression;

[0081] FIG. 5 shows a side view of a landing gear assembly according to a first embodiment of the invention after shock absorber compression;

[0082] FIG. 6 shows a flow chart of a method of detecting aircraft weight on wheels according to a second embodiment of the invention;

[0083] FIG. 7 shows a cross-sectional view of a movement detector according to a third embodiment of the invention;

[0084] FIG. 8 shows an enlarged cross-sectional view of the piston of the movement detector according to a third embodiment of the invention;

[0085] FIGS. 9 to 12 show sequential cross-sectional views of the movement detector according to a third embodiment of the invention during movement;

[0086] FIG. 13 shows a cross-sectional view of a movement detector according to a fourth embodiment of the invention;

[0087] FIG. 14 shows an enlarged cross-sectional view of the detachable body of the movement detector according to a fourth embodiment of the invention;

[0088] FIG. 15 shows a cross-sectional view of a movement detector according to a fifth embodiment of the invention;

[0089] FIG. 16 shows an enlarged cross-sectional view of the detachable body of the movement detector according to a fifth embodiment of the invention; and

[0090] FIGS. 17 to 19 show sequential cross-sectional views of the movement detector according to a fifth embodiment of the invention during movement.

DETAILED DESCRIPTION

[0091] FIG. 1 shows an aircraft 10 comprising a main landing gear 12, the aircraft being of a type that may be employed as the aircraft with which the methods and apparatuses of any of the illustrated embodiments may be used. The aircraft 10 thus includes a landing gear assembly 12 including a bogie, which is mounted on the lower end of the landing gear leg in such a way that the bogie may adopt different pitch angles.

[0092] FIG. 3 shows an aircraft landing gear assembly 112 according to a first embodiment of the invention. The landing gear assembly 112 comprises a shock absorber strut 114 comprising a piston 116 received within a cylinder 118. Cylinder 118 is connected to the airframe of an aircraft. The direction of the front of the aircraft is indicated by arrow F. Piston 116 is at its lower end pivotally connected to a bogie 120. The bogie 120 can thereby adopt different pitch angles relative the shock absorber strut 114. A pitch trimmer (not shown) controls the position of the bogie 120 relative to the shock absorber strut 114 in flight.

[0093] A plurality of wheels 122 are mounted on the bogie 120. In this embodiment three pairs of wheels 122a, 122b, 122c are mounted to bogie 120 by three axles. A link assembly 124 in the form of a torque link connects the cylinder 118 and the piston 116 of the shock absorber strut. The link assembly 124 comprises an upper arm 126 which is pivotally mounted to the cylinder 118 and a lower arm 128 which is pivotally mounted to the piston 116. The upper arm 126 and lower arm 128 are pivotally attached to each other at a hinge location. The link assembly 124 acts against rotational movement of the piston 116/bogie 120 relative to the cylinder 118/airframe. FIG. 3 also shows a second link assembly 130 in the form of a false link.

[0094] A movement detector 132 extends between the link assembly 124 and the bogie 120. One end of the movement detector is pivotally connected to the link assembly 124 at the hinge location. An opposing end of the movement detector 132 is pivotally connected to the bogie 120 proximate the aft end of the bogie 120.

[0095] The landing gear assembly 112 of the first embodiment has a trail angle of less than 10 degrees. During landing of the aircraft the aft pair of wheels 122a touchdown first. The bogie 120 subsequently pivots around the bottom of the shock absorber strut 114 until the centre 122b and front 122c pair of wheels have also touched down. At which point the bogie 120 is oriented substantially parallel to the ground G. In the present arrangement, the movement detector 132 is therefore compressed, as shown in FIG. 4.

[0096] Until the centre 122b and front 122c pair of wheels have touched down, there is unlikely to be enough aircraft weight going through the shock absorber strut 114 to cause it to compress. The link assembly 124 will therefore remain stationary relative to the airframe during this initial movement of the bogie 120 relative to the link assembly 124.

[0097] Thereafter, the shock absorber strut 114 begins to compress due to the weight of the aircraft. The link assembly 124 again moves relative to the bogie 120. The hinge location of the link assembly 124 moves aft and downwards. In the present arrangement this causes further compression of the movement detector 132, as shown in FIG. 5.

[0098] Compression of the movement detector 132 is detected by sensors, at least some of which being pressure transducers, in the movement detector 132. The sensors are in communication with a control system 134 of the aircraft. Upon compression of the movement detector, the sensors output a signal from which the control system 134 can determine that (i) there has been movement of the link assembly relative to the bogie and (ii) therefore there is aircraft weight on wheels.

[0099] In the event of a flat landing of the bogie 120, in which all pairs of wheels 122 touchdown at substantially the same time, it will be seen that movement is still detected due to shock absorber 114 compression, despite there being no or negligible pivotal movement of the bogie 120 about the shock absorber strut 114.

[0100] The aircraft may land with a negative trail angle, such that the front pair of wheels 122c touch down before the rear pair of wheels 122a. In this case the aft portion of the bogie 120 will initially pivot away from the link assembly 124. Thus the movement detector 132 extends in length until the bogie 120 is parallel to the ground. Subsequent shock absorber 114 compression then moves the link assembly 124 back towards the point on the bogie 120 where the movement detector is attached, thus causing compression of the movement detector 132. Both such movements could be used to detect aircraft weight on wheels, and could also be used to detect the time of shock absorber 114 compression.

[0101] In alternative embodiments the movement detector 132 may be mounted between the forward portion of the bogie 120 and the false link 130. In other alternative embodiments the movement detector 132 may be connected to the lower arm 128 below the hinge location.

[0102] A method 200 of detecting aircraft weight on wheels will now be described according to a second embodiment of the invention and with reference to FIG. 6. The method will be described with reference to an aircraft landing gear assembly according to the first embodiment.

[0103] The method begins subsequent to deploying (lowering) the aircraft landing gear from the aircraft wheel well. However the method may include a step of lowering the aircraft landing gear. The first step includes the control system 134 determining 202, from a radar altimeter, whether the altitude is below a predetermined value, in this example whether the altitude is below 10 feet. Provided the altitude condition is met, i.e. provided the altitude is below 10 feet, the control system 134 is configured to use the signal received from the movement detector 132 to determine whether there is aircraft weight on wheels. In embodiments in which the movement detector detects positon, the method may include and additional step of zeroing the movement detector and/or a step of taking a reading of the initial position of the movement detector (which corresponds to the initial position of the link assembly 124 relative to the bogie 120).

[0104] The method subsequently comprises a step of at least one wheel of the aircraft touching down 204 on the ground and concurrently the link assembly 124 moving 206 relative to the bogie 120. Depending on the orientation of the bogie 120 relative to the ground immediately prior to touchdown, and whether there is any equipment failures for example deflation of one or more of the tyres, the link assembly 124 moves relative to the bogie 120 by (i) the bogie 120 pivoting relative to the shock absorber strut 114 and/or (ii) the shock absorber strut 114 compressing thereby causing outward movement of the link assembly 124.

[0105] The method comprises a step of detecting 208 this movement using the movement detector 132. The movement detector 132 comprises a sensor in the form of one or more pressure transducers which are arranged to sense the occurrence of compression or extension of the movement detector 132 by detecting a transient change in pressure. The step of detecting 208 therefore comprises sensing compression or extension of the movement detector 132 using the one or more pressure transducers. Detecting 208 also comprises providing an output signal on the basis of which it can be determined that movement has occurred.

[0106] The method comprises a step of the control system 134 receiving 210 the signal output from the one or more pressure transducers of the movement detector 132. In this embodiment the control system 134 receives a nil or baseline signal when there is no compression or extension of the moment detector 132, and a different signal during compression or extension. In embodiments the movement detector may generate a single pulse upon movement. In other embodiments the control system may additionally receive a signal corresponding to position, for example a measurement of the travel of the ends of the movement detector.

[0107] Finally the method comprises a step of the control system 134 determining 212, on the basis of the signal received, that there is aircraft weight on wheels. In this embodiment aircraft weight on wheels is determined to have occurred when the signal received from the one or more pressure transducers departs from the baseline signal by a threshold amount.

[0108] The method of the second embodiment may be a part of a method of slowing an aircraft. In which case there is a subsequent step of deploying 214 at least one means of slowing the aircraft when the control system determines there to be aircraft weight on wheels.

[0109] A movement detector 332 according to a third embodiment of the invention will now be described with reference to FIG. 7. Movement detector 332 comprises a cylinder 336 having an internal space which houses a piston 338. The piston 338 divides the internal space into a first chamber 340 and a second chamber 342. The first chamber 340 and the second chamber 342 are filled with a hydraulic fluid. A hydraulic accumulator 350 keeps the hydraulic fluid in the first and second chambers 340, 342 topped up and at a substantially constant average pressure (PA).

[0110] The piston 338 is received on a piston rod 344 which extends through both end walls of the internal space. Two apertures 346, 348 are located at opposing ends of the movement detector. A first aperture 346 being located on the piston rod and a second aperture being located on the cylinder 336. The movement detector 332 is pivotally mountable to the bogie and the link assembly via the apertures 346, 348.

[0111] FIG. 8 shows an enlarged view of the piston 338. A flow restricted channel 352 extends through the piston 338 and puts the first chamber 340 into fluid communication with the second chamber 342. The flow restricted channel 352 comprises a first and a second restrictor 354, 355 in series which act to restrict the rate at which fluid can flow through the flow restricted channel 352.

[0112] A first non-return channel 356 connects the first chamber 340 with the flow restricted channel 352 at a point between the two restrictors 354, 355. The first non-return channel 356 contains a non-return valve 358 arranged to only permit fluid flow from the first chamber 340 into the flow restricted channel 352, not back again. A second non-return channel 360 connects the second chamber 342 with the flow restricted channel 352 at a point between the first restrictor 354 and the second restrictor 355. The second non-return channel 360 contains a non-return valve 362 arranged to only permit fluid flow from the second chamber 342 into the flow restricted channel 352, not back again.

[0113] A pressure transducer channel 364 runs through the piston rod 344 into the piston 338 and connects with the flow restricted channel 352 at a point between the first restrictor 354 and the second restrictor 355. A pressure transducer 366 is mounted to the piston rod 344 and is arranged to sense the pressure of the fluid in the pressure transducer channel 364. The pressure transducer 366 can be put in communication with a control system 334 comprising a signal processor.

[0114] A first pressure relief channel 368 comprising a first pressure relief valve 370 extends through the piston 338 to permit fluid flow from the first chamber 340 into the second chamber 342 when the pressure in the first chamber 340 exceeds a threshold pressure (the crack pressure, P.sub.C) relative to the pressure in the second chamber 342. A second pressure relief channel 372 comprising a second pressure relief valve 374 extends through the piston 338 to permit fluid flow from the second chamber 342 into the first chamber 340 when the pressure in the second chamber 342 exceeds a threshold pressure relative to the pressure in the first chamber 340.

[0115] FIGS. 9 to 12 show how movement is detected upon movement of the piston 338 in the cylinder 336. FIG. 9 shows the movement detector 332 is prior to movement. The piston 336 is positioned in the middle of the cylinder 338 such that the first chamber 340 and second chamber 342 have approximately equal volumes (although it will be understood that the piston need not start in this position). The fluid in the chambers 340, 342 is held at the pressure of the accumulator.

[0116] FIG. 10 shows the piston 336 having moved in the direction of the first chamber 340. Such movement corresponds to an increase in length of the movement detector 338. In use, for example in the arrangement shown in FIG. 3, such movement is due to a movement of the bogie 120 relative to the link assembly 124. The reduction in volume of the first chamber 340 leads to an increase in the pressure of the fluid held therein. The first non-return valve 558 opens and fluid therefore flows into the flow restricted channel 352 via the first non-return channel 356 and (to a limited extent) via the first flow restrictor 354. The pressure of fluid in the pressure transducer channel 364 thus also increases. The pressure increase is detected by the pressure transducer 366 as shown in the graph 378 of pressure detected vs time.

[0117] The flow restrictors 354, 355 limit the rate of flow between the chambers such that the pressure of the fluid can build up to a level which is detectable by the transducer. If the restrictors 354, 355 do not limit flow rate between the first and second chambers 340, 342 enough, then then the pressure in the first chamber 340, and thus the pressure in the pressure transducer channel 364, may not increase to a level which is readily detectable by the transducer 366, even upon a fairly rapid movement of the piston 336. Conversely, if the flow is restricted too much, then the pressure may build up rapidly upon even slight movements of the piston 338 in the cylinder 336, such as may be caused by vibrations in the landing gear assembly. The rate of pressure build up may be affected by several other variable such as the compressibility of the fluid and the chamber size.

[0118] FIG. 11 shows the piston 336 having moved further in the direction of the first chamber 340 such that the pressure in the first chamber 340 has exceeded the threshold pressure of the first pressure relief valve 370. The diameter of the pressure relief channels 368, 372 exceeds the diameter of the flow restrictors 354, 355 and is such that the pressure difference between the first and second chambers 340, 342 can equalise more quickly than via the flow restricted channel 352 alone. This also allows the piston to move more quickly within the cylinder. In use this may reduce the forces and stress on the movement detector caused by rapid movement of the bogie upon touchdown. Should the threshold pressure of the first pressure relief valve 370 not have been met, the pressure would be left to equalise via the flow restricted channel 352 only. FIG. 12 shows the movement detector in its new position following the movement.

[0119] In the movement detector 332 high pressure fluid contained in the flow restricted channel 352 between the restrictors 354, 355 cannot quickly equalise via the first pressure relief channel 368 because the previously open first non-return valve 358 will close. Thus the fluid must flow out at a slower rate via the restrictors 354, 355. The pressure measured by the pressure transducer 366 therefore decays more slowly than if the pressure in the first chamber 340 was measured directly.

[0120] It will be appreciated that should the link assembly move relative to the bogie in the opposite direction, such that the piston moves in the direction of the second chamber, the process described above will repeat except it will be the second non-return valve 362 which opens to permit fluid to flows into the flow restricted channel 352, and the second pressure relief valve 374 which opens to allow the pressure to more quickly equalise.

[0121] When connected to an aircraft control system the control system may determine that movement is significant enough to be caused by aircraft weight on wheels only when the measured pressure exceeds a threshold pressure (P.sub.T). The threshold pressure is preferably between the accumulator pressure (P.sub.A) and the threshold pressure of the pressure relief valve (P.sub.C). In alternative embodiments the movement detector may comprise its own signal processor which provides a discrete true/false signal while the pressure exceeds the threshold pressure. The aircraft control system may use the receipt of the signal to determine that there is aircraft weight on wheels.

[0122] A movement detector 432 according to a fourth embodiment of the invention will now be described with reference to FIG. 13. Like the third embodiment, a cylinder 436 houses a piston 438 which divides an internal space of the cylinder 436 into a first chamber 440 and a second chamber 442. The piston 438 is received on a piston rod 444 which extends through both end walls of the internal space. Apertures 446, 448 on the piston rod and cylinder can be used to mount the movement detector 432 to a landing gear assembly.

[0123] In the fourth embodiment the movement detector comprises a detachable body 476 which is in fluid communication with the first chamber 440 and second chamber 442 by inlet/outlet ports proximate the end walls of the chambers 440, 442. The body 476 comprises a similar arrangement of valves and channels to the piston 336 of the third embodiment. Fluid flows through the valves in response to movement of the piston in a similar way to how fluid flows in the third embodiment. The difference being that movement of the piston 438 in this fourth embodiment forces fluid out of one chamber into the other chamber via the body rather than via the piston.

[0124] FIG. 14 shows an enlarged view of the body 476. A flow restricted channel 452 extends through the body 476 and puts the first chamber 440 into fluid communication with the second chamber 442. The flow restricted channel 452 comprises a first and a second restrictor 454, 455 in series which act to restrict the rate at which fluid can flow through the flow restricted channel 452.

[0125] A first non-return channel 456 comprising a first non-return valve 458 connects the first chamber 440 with the flow restricted channel 452 at a point between the two restrictors 454, 455. A second non-return channel 460 comprising a second non-return valve 462 connects the second chamber 442 with the flow restricted channel 452 at a point between the first restrictor 454 and the second restrictor 455.

[0126] A pressure transducer channel 464 runs into the top of the body 476 and connects a pressure transducer 466 with the flow restricted channel 452 at a point between the first restrictor 454 and the second restrictor 455. The pressure transducer 466 can be put in communication with a control system 434 comprising a signal processor.

[0127] A first pressure relief channel 468 comprising a first pressure relief valve 470 extends between the inlet/outlet ports of the body 476 to permit fluid flow from the first chamber 440 into the second chamber 442 when the pressure in the first chamber 440 exceeds a threshold pressure (the crack pressure) relative to the pressure in the second chamber 442. A second pressure relief channel 472 comprising a second pressure relief valve 474 extends between the inlet/outlet ports of the body 476 to permit fluid flow from the second chamber 442 into the first chamber 440 when the pressure in the second chamber 442 exceeds a threshold pressure relative to the pressure in the first chamber 440.

[0128] A hydraulic accumulator 450 keeps the hydraulic fluid in the first and second chambers 440, 442 topped up and at a substantially constant average pressure by feeding into the body 476, rather than directly into one of the chambers.

[0129] A movement detector 532 according to a fifth embodiment of the invention will now be described with reference to FIG. 15. The piston 538 and cylinder 536 are all arranged as per the fourth embodiment of the invention. A detachable body 576 comprises channels and valves which connect the first and second chambers 540, 542. A hydraulic accumulator 550 connects into the body 576 as per the fourth embodiment.

[0130] FIG. 16 shows an enlarged view of the body 576. The body 576 comprises a flow restricted channel 552 comprising a single restrictor 554. A first and a second transducer 566, 567 are arranged to measure the pressure of the fluid either side of the restrictor 554. Similarly to the fourth embodiment, two pressure relief channels 568, 572, having pressure relief valves 570, 574, extend between the inlet/outlet ports of the body 576.

[0131] FIGS. 17 to 19 show how movement is detected upon movement of the piston 538 in the cylinder 536. FIG. 17 shows the movement detector 532 prior to movement. The piston 536 is positioned in the middle of the cylinder 538 such that the first chamber 540 and second chamber 542 have approximately equal volumes (although it will be understood that the piston need not start in this position). The fluid in the chambers 540, 542 is held at the pressure of the accumulator 550.

[0132] FIG. 18 shows the piston 536 having moved in the direction of the first chamber 540. Such movement corresponds to an increase in length of the movement detector 538. In use, for example in the arrangement shown in FIG. 3, such movement is due to a movement of the bogie 120 relative to the link assembly 124.

[0133] The reduction in volume of the first chamber 540 leads to an increase in the pressure of the fluid held therein. The fluid therefore flows into the flow restricted channel 552. The flow restrictor 554 limits the rate of fluid flow into the second chamber 542 which therefore causes the pressure in the first chamber 540 to build up. The increase in pressure is measured by the first transducer 566. Conversely, the increase in volume of the second chamber 542 leads to a decrease in the pressure of the fluid held therein. The decrease in pressure is similarly detected by the second transducer 567. Graphs 578 show the pressure detected by the transducers vs time (the solid line corresponding to the first transducer 566 and the dashed line corresponding to the second transducer 567).

[0134] The use of two pressure transducers 566, 567 makes it possible to detect the direction of movement of the piston 538 within the cylinder 536, and therefore direction of movement of the bogie relative to the link assembly.

[0135] FIG. 19 shows the piston 536 having moved further in the direction of the first chamber 540 such that the pressure in the first chamber 540 has exceeded the threshold pressure (PC) of the first pressure relief valve 570 thereby allowing fluid to flow through the first pressure relief channel 568. The pressure difference between the first and second chambers equalises more quickly via the first pressure relief channel 568. The pressure detected by the pressure transducers is likely to return to the accumulator pressure (PA) faster than in the third and fourth embodiments as the fluid whose pressure is being measured does not get trapped between two restrictors.

[0136] When connected to an aircraft control system the control system may determine that movement is significant enough to be caused by aircraft weight on wheels only when the measured pressure exceeds a threshold pressure (PT). The threshold pressure is preferably between the accumulator pressure (PA) and the threshold pressure of the pressure relief valve (PC). In alternative embodiments the movement detector may comprise its own signal processor which provides a discrete true/false signal while the pressure exceeds the threshold pressure. The aircraft control system may use the receipt of the signal to determine that there is aircraft weight on wheels.

[0137] It will again be appreciated that should the link assembly move relative to the bogie in the opposite direction, such that the piston moves in the direction of the second chamber, the process described above will repeat except it will be the second transducer 567 which detects an increase in pressure, the first transducer 566 which detects a decrease in pressure, and the second pressure relief valve 574 which opens to allow the pressure to more quickly equalise.

[0138] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. Some examples of such variations will now be described by way of example only.

[0139] In another embodiment of the invention there is provided a movement detector similar to that described in relation to the fifth embodiment. Instead of the hydraulic network being located in the body, the hydraulic network is located in the piston. In another embodiment of the invention, the movement detector additionally detects movement of the piston within the cylinder, and therefore movement of the link assembly relative to the bogie, by using a sensor which is arranged to detect the flow of fluid (for example by detecting fluid flow speed) through one or more of the channels. In embodiments where the fluid is substantially incompressible there may be very little or no compression (reduction in volume) of the fluid when force is applied to the piston as a result of aircraft weight on wheels. Thus the piston may move very little, and only at a rate corresponding to the rate at which fluid can flow through the flow restricted channel, until the pressure relief valves open to allow fluid to flow more quickly between the chambers.

[0140] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.